Japanese researchers from the University of Tokyo and the National Museum of Nature and Science in Ibaraki have precisely measured the brain size of Homo floresiensis, a dwarfed human species that lived between 95,000 and 18,000 years ago on a remote Indonesian island.

The first high-resolution micro-CT scans of the only remaining Homo floresiensis skull reveal that the brain size was 426 cubic centimeters. This is larger than the previously cited figure of 400 cc, although still much smaller than that of modern humans – 1300 cubic centimeters on average.

The Japanese team also analyzed 20 different modern human populations worldwide and showed that the relationship between body size and brain size is actually stronger than previously suggested.

The precise measurements allowed the team to construct new models of the brain size reduction in the evolution of Homo floresiensis.

“The new model shows that, contrary to expectations by some researchers, it is possible that large-bodied Homo erectus migrated to a solitary island and evolved into Homo floresiensis by marked island dwarfism,” said lead author Dr Daisuko Kubo and his colleagues.

Previous studies have differed as to whether Homo floresiensis descended from small-bodied and small-brained early Homo habilis (lived 2 millions years ago) or from large-bodied and large-brained Homo erectus (lived 1.7-0.05 million years ago). However, there is a lack of fossil evidence for the presence of Homo habilis in Asia, where the Homo floresiensis resided, suggesting they instead evolved from Homo erectus.

The theory that Homo floresiensis evolved from Homo erectus, as modern humans did, was also previously thought untenable, due to the mean brain size of Homo erectus being very large – about 991 cubic centimeters, and thus requiring an extreme evolutionary brain size reduction to that of the Homo floresiensis.

However, according to the new results, not only was the Homo floresiensis brain size larger than previously thought, but the earliest Homo erectus’ brain size was around 860 cubic centimeters rather than the previously cited 991 cubic centimeters.

“Thus Homo erectus is the most appropriate ancestor for the Homo floresiensis in this model,” the scientists concluded.